1. Field of the Invention
The present invention relates generally to door assemblies and, more specifically, to an improved hangar door assembly.
2. Desription of Related Art
A continuing hazard in aircraft hangars and other large buildings has been the doors, which are of necessity large and heavy, covering the entrances and exits to such structures. The doors present hazards to an operator either opening or closing the doors, as well as persons that may be in the immediate area while the doors remain open or closed. Because of their heavy weight, the doors create a danger of inadvertently moving between their open and closed positions. A free swinging door, especially one being drawn downward by gravitational forces, could easily cause injuries to persons in the area. Furthermore, a bifolding door that is drawn up by cables and down by gravity is subject to inadvertent movement by cable or driving motor failure. The large size of the doors also present safety hazards in an environment of wind gusts, such as in airfields having aircraft hangars. The surface areas of the hangar doors create large areas of wind resistance. In the presence of frequent and strong wind gusts, the doors can be dislodged from their fully open or partially open positions and create a significant risk of injury to person and property.
The design of some of these doors also present space problems, particularly when the door must remain in an open position. Frequently, the size of the door is almost equal to that of a side of a building. In such case, if the door is disposed within the building while in an open position, a large amount of interior useable space is lost. If the door is designed to remain exteriorly of the building while open, the problem of wind gusts as noted above, then becomes a concern.
One example of a door assembly for large passageways is found in Aspenson, "Canopy Door," U.S. Pat. No. 4,124,055. Aspenson provides a series of cables, pulleys, and extendable struts to lift and lower the door. One problem with such a design is the apparent stress upon the pulley system which could become weakened by the weight of the door. A further drawback is the space which is taken up interiorly of the building structure when the door is in its fully open position.
Another example of a door for a large passageway is found in Doering, "Greenhouse Door Assembly," U.S. Pat. No. 4,120,341 wherein a handle is fixed centrally of the door with a trailing rope tied to one end of the handle. An operator uses the handle to lift and lower the door, as a pulley system and counter weights balance the weight of the door. The ability to open and close the door would appear to be inherently limited by the arm's reach of an operator. Another apparent problem is that the lower portion of the door appears to remain free swinging. This may present a risk of injury to the user and those in the vicinity.
Another design is disclosed in Heier, "Foldable Wall Assembly," U.S. Pat. No. 3,299,740. A foldable door, which is apparently relatively small, remains in a closed position interiorly of the housing structure and consequently presents the type of space problem described above. Buehler, "Door Operators," U.S. Pat. No. 3,389,740 discloses a door system that utilizes a pair of linkage arms connected to a shaft. The shaft is rotated by a swing arm that is connected to a follower and screw, the latter being rotated by an electric motor. A problem with this design is that it is apparently directed towards small, light-weight doors, such as that used for car garages.
A need still exists in the art for an improved door assembly that is effective in connection with large, heavy doors.